In a cellular communication system, a geographical region is divided into a number of cells served by base stations. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile station is served via a radio communication link from the base station of the cell within which the mobile station is camped on. Communication from a mobile station to a base station is known as the uplink, and communication from a base station to a mobile station is known as the downlink.
The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Internet or the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication, etc.
Currently, 3rd generation systems are being rolled out to improve the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) technology. Both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) techniques employ this CDMA technology. In CDMA systems, user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency and in the same time intervals.
In a 3rd generation cellular communication system, the communication network comprises a core network and a Radio Access Network (RAN). The core network is operable to route data from one part of the RAN to another, as well as interfacing with other communication systems. In addition, it performs many of the operation and management functions of a cellular communication system, such as billing. The RAN is operable to support wireless user equipment over a radio link of the air interface. The RAN comprises the base stations, which in UMTS are known as Node Bs, as well as Radio Network Controllers (RNC) which control the base stations and the communication over the air interface.
The RNC performs many of the control functions related to the air interface including radio resource management and routing of data to and from appropriate base stations. It further provides the interface between the RAN and the core network. An RNC and associated base stations are collectively known as a Radio Network Subsystem (RNS).
3rd generation cellular communication systems have been specified to provide a large number of different services including efficient packet data services. For example, downlink packet data services are supported within the 3GPP release 5 specifications in the form of the High Speed Downlink Packet Access (HSDPA) service. In accordance with the 3GPP specifications, the HSDPA service may be used in both Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode. The HSDPA service may be used to support Voice over Internet Protocol (VoIP) services as well as Push-to-Talk (PTT) services and the like.
In UMTS systems that support HSDPA, transmission code resources are managed in both the RNC and the Node B. The base station (also known as the Node-B for UMTS) is responsible for allocating and distributing the shared HSDPA code resources to the users who have a High Speed Downlink Shared CHannel (HS-DSCH) assigned. The RNC is responsible for allocating code resources to Dedicated CHannels (DCH's) and other common channels. Hence, in UMTS systems that support HSDPA, some code resource allocation is performed by the RNC whereas other code resource scheduling is performed by the base station. Specifically, the RNC allocates a set of resources to each base station, which the base station can use exclusively for high speed packet services. The base station is responsible for scheduling transmissions on the HS-DSCH to the mobile stations that are attached to it, for operating a retransmission scheme, for controlling the coding and modulation of HS-DSCH transmissions to the mobile stations, and for transmitting data packets to the mobile stations.
HSDPA seeks to provide packet access techniques with a relatively low resource usage and with low latency. Specifically, HSDPA uses a number of techniques in order to reduce the resource required to communicate data and to increase the capacity of the communication system. These techniques include Adaptive Modulation and Coding (AMC), retransmission with soft combining and fast scheduling performed at the base station. It is important to provide fast scheduling during handover and low latency for real time services such as VoIP conversational services.
Although 3rd Generation cellular communication systems support soft handover wherein transmissions between a mobile station and a plurality of base stations are combined for improved performance, HSDPA is designed for only a single cell. Accordingly, HSDPA relies on only a single radio link and soft handover of HSDPA signals is not supported. Thus, in an HSDPA enabled cellular communication system some communication channels may support soft handover whereas other communication channels (such as HSDPA channels) do not, which results in delays when selecting cells during handover.
As a result, on the mobility enhancement for the VoIP/HSDPA, there are currently two concerns: a) handover delay, which is a general concern for the VoIP traffic characteristics, wherein the delay for cell change can be much higher, and b) handover message transmission reliability in the serving cell, which is unacceptable for real time services. Message reliability in the serving cell is critical for real time services as failure to reliably deliver the handover message to the mobile station before the mobile station moves out of the cell will result in a dropped call.
A general solution to the handover delay problem is to pre-configure certain cells (or possibly a subset of cells) of the active set and the UE with HSDPA related configurations. The subset of the active set for the pre-configurations is termed HSDPA active subset, which contains cells that can provide HSDPA service to the UE. However, this is only a partial solution for reducing cell change delay, and is still not sufficient for real-time services.
One non-optimal specific solution uses unilateral UE-based signalling wherein the UE reports to the network about the best cell for the handover. The UE performs measurements on the current serving cell and on the cells of the HSDPA active subset, as earlier preconfigured by the network, and keep track of the current best cell. If one of the cells of the HSDPA active subset becomes the best cell then the UE sends a report message to the network. However, there is a difference in the way the UE reports.
In particular, this non-optimal solution uses a Layer 2 signalling based reporting technique, wherein the Medium Access Control (MAC-e) header is used for the UE to identify the best cell. However, there are two disadvantages in this solution; a) the HSDPA real-time service support relies on High Speed Uplink Packet Access (HSUPA) deployment, which may not be available; and if there is no HSUPA simultaneously, the procedure cannot be performed. Furthermore, a change to the MAC-e header is anticipated, and b) during the cell change procedure, if there is no MAC-e Protocol Data Unit (PDU) generated, one or multiple empty MAC-e PDUs needs to be generated in order to transmit the MAC-e header for the cell change purpose. Moreover, the reliability of the signalling needs additional actions such as power boosting of particular MAC-e PDU.
In this non-optimal solution, the UE performs the cell change. However, unilateral switching to listen to the best cell is used without any acknowledgement from the network. Although the activation time can be used to synchronize the UEs and the Node Bs, the activation time is set by the UE itself, not by the network side which is risky. For example, if the activation time is set unnecessarily long, the call may be dropped due to the poor radio condition of the serving cell. If the activation time is set too short, the user may lose some data due to its early switching.
Another non-optimal specific solution uses unilateral UE-based signalling with a Layer 1 signalling based technique. By using the FeedBack Information (FBI) bits on the uplink, the UE can signal the best cell for the selection. In this non-optimal solution, an implicit switching mode is applied with the capability that the RNC can override it. During the transient switching period, the UE will monitor two High Speed Shared Control Channels (HS-SCCHs) from the serving cell and two HS-SCCHs from the target cell directly before any acknowledgment from the network. The UE also reports the Channel Quality Indicator (CQI) for the serving cell in an odd number of Transmission Time Intervals (TTIs) and the CQI for the target cell in even number of TTIs. After the UE receives the HS-SCCH scheduling from the specific cell, the UE will report the CQI to the specific cell which scheduled it. In this technique, the UE should monitor the HS-SCCHs from two cells, and report the CQI for two cells during the switching period. Furthermore, it should be noted that the channel timing for two cells is different which is related to the cell specific Common Pilot Channel (CPICH) timing. As a result, it is difficult to have the common synchronized timing from the UE side for the reporting or monitoring (even TTI for the serving cell and odd TTI for the target cell).
In both specific non-optimal solutions, the UE performs the switching before any network acknowledgment. Although this is good from the point of view of delay, it is not good for robustness and simplicity. Autonomous, unacknowledged switching by the UE is undesirable in that the UE may choose a cell that is unacceptable or suboptimum to the network. In particular, real network conditions are complicated; hence it is difficult and risky to predict the RNC (Radio Network control) will always accept the switching. As a result, the UE's unilateral switching carries the risk that if the network rejects the switching a much longer delay and possible drop of the call is anticipated.
A general solution to the handover reliability problem is to include: a) parallel monitoring of source and target node-b HS-SCCHs, and b) implicit handover to target node-b at first scheduling occurrence. However, there are several disadvantages associated with these techniques involving complexity and flexibility.
As for complexity, the UE needs to monitor the HS-SCCH from both the serving cell and the target cell. This puts complexities from the RF aspects for the UE receptions. Further, in this case, the HS-SCCH allocation should be very careful. If there are no scheduling events on the target cell HS-SCCH, the UE needs to switch back and find another one. Note that there is no CQI feedback for the initial transmissions, which will reduce the performance. Also note that the ACK/NACK transmissions to the target cell at the initial stage will become very difficult because there is no power control for the uplink HS-DPCCH initially which needs power control command. Moreover, the simultaneous reception of two HS-DSCH from two different cells may be required during the handover period.
As for flexibility, the UE assumes the network always accepts its request. Based on that, the UE performs the implicit handover without any handoff message guidance from the network. However, since the real network conditions are complicated, it is difficult and risky to predict that the RNC will always accept the switching. In addition, the UE's unilateral switching may have the risk that the network rejects the switching, where a much longer delay and possible drop of the call is anticipated.
What is needed is a system and method to alleviate the aforementioned problems.